Method of using heat sensitive copying paper



April 3, 1956 c. s. MILLER 2,740,896

METHOD OF USING HEAT SENSITIVE COPYING PAPER Filed May 10. 1947 Source of rad/42700.

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fiavemiar .7 MMMvW United States Patent METHOD OF USING HEAT SENSITIVE COPYING PAPER Carl S. Miller, St. Paul, Minn., assignor to Minnesota Mining & Manufacturing Company, St. Paul, Minn, a corporation of Delaware Application May 10, 1947, Serial No. 747,338

11 Claims. (Cl. 250-65) This invention relates to a method for the reproduction of printed matter or the like, and to novel sheet material useful therein. The method involves the application of intense radiant energy to the graphic subjectmatter of which a copy is desired, the conversion of radiant energy to heat energy in a pattern determined by such subject-matter, and the formation of visible copy in the sensitized sheet material as a result of the localized heating thus obtained.

The invention has particular utility in affording a means for rapidly obtaining one or more copies of printed matter, diagrams, photographs, or other graphic subjectmatter directly from the original, as in conducting library searches or the like. There is involved merely the proper positioning of the sensitive copy-paper with respect to the original, and the exposure of the original to intense radiant energy. A true copy is produced directly, without the necessity of subsequent development of a latent image or of other processing.

Radiant energy of an intensity suflicient for the process of this invention may be obtained from any suitable source. One convenient source of radiation, which has given good results over limited areas, consists of a standard SOO-watt infra-red bulb with internal reflector, operated under overload conditions of 800 watts input, and at a distance of about three inches from the printed surface. Photo-flash bulbs may be used for irradiating small areas. Other sources having a still higher output of radiant energy, particularly at wave-lengths of less than about 25,000 angstroms, and capable of uniformly irradiating larger areas, are even more suitable.

My novel sensitized sheet material or copying-paper consists of a transparent sheet material carrying a heatsensitive composition capable of undergoing irreversible visible change when momentarily heated to a predetermined temperature.

In the drawing, which is not necessarily to scale, the transparent copying-paper is held in close and heatconductive contact with a portion of printed page 11 of book 12, by means not shown, but which may consist of a flat transparent plate of material having low heat conductivity at least at the surface in contact with the copying paper. A plate of Pyrex glass with an interposed layer of open-mesh silk bolting-cloth, placed over the transparent copying-paper, has given good results. Radiant energy from source 13, conveniently an electric infra-red lamp bulb operated as hereinbefore described, passes through the transparent copying-paper 10 (as well as through the transparent glass plate or other holding means Where such is employed), and falls on the printed characters below, resulting in a permanent visible duplication of such characters in the heat-sensitive transparent copying-paper. Obviously, the strip or sheet of copying-paper 10 may be of any size and shape depending on the extent of the material of which a copy is desired, e. g. it may cover the entire page 11. The portions of the copying-paper not directly above the printed characters during this exposure are found to remain substantially unchanged in appearance. Viewed from above, i. e. from the surface not in contact with the printed page, the thus exposed copying-paper shows a facsimile copy of the printed or other graphic subject-matter of the original. It will be obvious that a reverse copy is to be seen when the sheet is viewed from below.

In probable explanation of this effect, it is believed that the radiation falling on the printed characters is absorbed thereby and converted to heat, resulting in an elevated-temperature pattern corresponding to such printed characters, which in turn produces a visible change in the heat-sensitive material of the transparent copying-paper. On the unprinted portions of the page, very little of the radiation is absorbed and converted to heat; thus the temperature does not increase at such areas, at least not to a degree comparable to that in the inked areas, and the copying-paper remains unaffected. in support of this theory, it has been observed that the rapid withdrawal of heat during irradiation, as by contact of the copying-paper with a highly heat-conductive layer, is sufiicient to retard or prevent the development of a visible image on the heat-sensitive sheet. For example,

the use of a smooth plate of glass at room temperature and in direct contact with the upper surface of the copying-paper, as a holding means therefor, has been observed to interfere with the production of a desirably clear copy;

The type of ink or other marking means of which the printed characters are composed, and the wave-length ofithe radiation employed, are also found to have an influence on the appearance of the copy. Carbon inks are generally quite absorbent toward visible or infra-red light and produce excellent copies when so illuminated or irradiated. Other inks may give inferior results with certain wave-lengths but may respond selectively to specific wave-lengths produced by sources of specific radiation or isolated from other sources by means of proper filters.

For the process as above described, it will be observed that my novel copying-paper must be quite transparent to the particular wave-length employed, i. e. the radiation used must be able to penetrate readily through the copying-paper Without excessive diffusion or absorption. The copy must be viewed from the surface not initially in contact with the original, i. e. from the exposed surface as shown in the drawing, in order to avoid a reversal of the message. The intensity of the high-temperature pattern in the copying-paper during exposure is greatest at the surface in contact with the original, and ordinarily the copying-paper will be designed to visibly reproduce the pattern at or near its surface. It is therefore desirable to employ a visibly transparent sheet so as to obtain maximum readability. Furthermore, visible light will ordinarily be the most convenient radiation. However, sheet material which appears somewhat translucent or even opaque to the eye may still be effectively transparent in regard to suitable radiation of other wave-lengths, and may still be found to function as a useful copying-paper when employed according to my novel method.

it will also be apparent that the heat-sensitive composition of my transparent copying-paper must be reason ably stable toward light, i. e. must not be rapidly converted to a visibly diiferent form by exposure to light alone. Even a gradual reaction in the presence of actinic light at high intensity is undesirable where permanent copies are sought, and substantially completely light-stable compositions are generally preferred, although considerable latitude in this respect is obviously permissible where copies are intended for temporary use only.

For best results in the reproduction of printed matter, and particularly of half-tone prints or the like which contain areas of fine detail as well as massive darkened areas, certain requirements as to time and temperature of exposure or activation may be noted. The heat-sensitive composition must be capable of changing to a visibly distinct form having a desirably high contrast number with a minimum exposure and over a minimum range of temperature. Compositions which are converted from the transparent to the opaque form instantaneously on heating to a specific temperature would be highly desirable for many purposes. However, somewhat slower-reacting compositions which provide a contrast number of at least about 0.4 within less than one-half second, or preferably less than one-tenth second, and which furthermore provide this entire degree of darkening within a temperature range of not more than about 20 C, and preferably not more than about 5 C., have been found to provide reproductions of good detail and contrast.

In determining the contrast number, suitably exposed samples having printed (darkened) and unprinted (undarkened) areas are held against a fiat magnesium carbonate block (a standard magnesium carbonate surface as defined in the paint industry). A beam of white light (north exposure daylight) is directed against the outer surface of the paper at an angle of 45, and the intensity of the light reflected normal to the surface is measured by means of a suitable photometer. The contrast number is defined as the following ratio Iu-Id I'LL where I represents intensity, and subscripts :1 (I refer to measurements made over the undarkened area and darkened area, respectively. In determining the contrast number the samples may conveniently be exposed, at specific temperatures and for specific time intervals as required, by pressing them against heated metal plates.

Although in my copying process the transparent sheet may be fullyexposed to visible light, e. g. as shown in the drawing, this is not essential, since sensitizing compositions may be employed which will provide a visible change even though the coated copying-paper is previously stored and maintained in complete darkness, and finally exposed to my copying-process in the complete absence of visible light.

Most of the more desirable backings or sheet materials which I may employ are cellulosic, or at least organic in character, and hence are subject to deterioration when heated excessively. i therefore normally prefer to select sensitizing compositions which provide a visible change at a temperature of somewhat less than about 150 C., or even as low as about 5060 C. However, other materials and other applications in some cases make it desirable to employ compositions which change at sub- ,antia ly higher temperatures. Farticularly in such cases, .emplate raising the temperature of the printed surand of the transparent copying-paper, prior to irradiation, to a point above room temperature but below the reaction temperature, in order to permit the use of radiation of substantially lower intensity than would otherwise be required.

"The following examples provide specific transparent temperature-sensitive copying-papers which have been found to produce good results when employed in the reproducing of printed matter in accordance with the procedure described. However, it will be understood that the invention is not to be limited to these specific examples, since various equivalent compositions and backing materials alternatively be employed.

Example I A strip of cellophane was passed through a solution prepared by mixing saturated aqueous solutions of the following materials:

Volumes Nickel acetate s 50 Nickel nitrate Ammonium thicsulfate Thiourea 40 Cit The strip was drained and was allowed to dry at room temperature. The dried sheet was fully transparent, but became black and opaque when heated to about 120 C. for less than one-half second, and when employed in the process as heretofore described was found to be useful for the production of copies of graphic subject-matter.

Example 2 Cellophane was treated with an aqueous solution containing. in parts by weight, 10 parts of lead acetate, 10 parts of thiourea, and 0.1 part of acetic acid in parts of water, and was dried as in Example 1. A transparent heat-sensitive copying-paper was produced.

Example 3 Thin unsized flax paper was coated and impregnated with a mixture of finely divided dispersed particles of methylene blue and of mercuric stearate in a xylol solution of ethyl cellulose, ()n drying, the sheet was quite transparent, e. g. when the sheet was held in contact with a printed page, the printing could be seen through the sheet; it had a faint blue color, and was unchanged on prolonged exposure to light. When used as a transparent heat-sensitive copying-paper, it produced an intense blue facsimile copy of the graphic original. The sheet was most satisfactorily exposed with the coated surface in contact with the original printed page, but usefully clear and exact copies could alternatively be prepared with the untreated surface in contact with such original, or even with the copying-paper suitably supported against the reverse side of the printed page. In this last-named modification, the thickness of the Web supporting the heat-sensitive surface coating could be considerably increased.

The use of these or other heat-sensitive compositions, together with suitable transparent binders or carriers where desired, as coatings on various transparent base sheet materials or in self-sustaining films is also contemplated.

Specific examples of transparent copying papers and of methods for their use in the preparation of facsimile copies of graphic subject-matter have been given, together with a discussion of the principles involved, all in a manner designed to render the invention fully understandable to those skilled in the art. It will be understood that all equivalent modifications are likewise apprehended.

Having now described my invention and illustrated the same with specific examples, what I claim is as follows:

1. A method for the production of permanent facsimile copies of a graphic original having a pattern consisting of portions highly absorptive of radiant energy, said radiant energy on absorption thereby being converted to heat energy, and other portions sufficiently less absorptive of said radiant energy so that, on irradiation of said original as hereinafter provided, only the said highly absorptive portions will attain a temperature sufiicient to cause a visible change in an associated heat-sensitive copying-surface; said method comprising: (a) placing said graphic original in heat-conductive association with a heatsensitive copying-surface which changes visibly when heated, and (1)) strongly and briefly exposing said graphic original to said radiant energy to provide thereon an elevated-temperature pattern corresponding to the absorptive portions of said graphic original and of an intensity adequate to cause a visible change in said associated copying-surface.

2. A method for the production of permanent facsimile copies of a graphic original having a pattern consisting of portions highly absorptive of radiant energy, said radiant energy on absorption thereby being converted to heat energy, and other portions sufiiciently less absorptive of said radiant energy so that, on irradiation of said orginal as hereinafter provided, only the said highly absorptive portions will attain a temperature suiiicient to cause a visible change in an associated heatsensitive copying-surface; said method comprising: (a) placing said graphic original in contact with a separate heat-sensitive copying-layer which exhibits a permanent visible change upon exposure to heat, (b) strongly and briefly exposing said graphic original to said radiant energy to provide thereon an elevated-temperature pattern corresponding to the absorptive portions of said graphic original and of an intensity adequate to cause a visible change in said copying-layer, and (c) separating said original and said copying-layer.

3. A method for the production of permanent facsimile copies or" a graphic original having a pattern consisting of portions highly absorptive of radiant energy, said radiant energy on absorption thereby being converted to heat energy, and other portions sufliciently less absorptive of said radiant energy so that, on irradiation of said original as hereinafter provided, only the said highly absorptive portions will attain a temperature sufiicient to cause a visible change in an associated heatsensitive copying-surface; said method comprising: (a) placing said graphic original in contact with a separate heat-sensitive copying-paper transparent to said radiant energy and which exhibits a permanent visible change upon exposure to heat, ([2) strongly and briefly exposing said graphic original to said radiant energy through said transparent copying-paper to provide on said graphic original an elevated-temperature pattern corresponding to the absorptive portions of said graphic original and of an intensity adequate to cause a visible change in said contacting copying-paper, and (c) separating said original and said copying-paper.

4. A method for the production of permanent facsimile copies of a graphic original in the form of a thin sheet material having on one surface thereof a pattern consisting of portions highly absorptive of radiant energy, said radiant energy on absorption thereby being converted to heat energy, and other portions sufliciently less absorptive of said radiant energy so that, on irradiation of said original as hereinafter provided, only the said highly absorptive portions will attain a temperature suflicient to cause a visible change in an associated heatsensitive copying-surface; said method comprising: (a) placing said graphic original with its other surface in contact with a separate heat-sensitive copying-surface which changes visibly when heated, (b) strongly and briefly exposing said one surface of said graphic original to said radiant energy to provide in said thin sheet material an elevated-temperature pattern corresponding to the absorptive portions of said graphic original and of an intensity adequate to cause a visible change in said copyingsurface, and (c) separating said original and said copying-surface.

5. A method for the production of permanent facsimile copies of a graphic original having a pattern consisting of portions highly absorptive of radiant energy, said radiant energy on absorption thereby being converted to heat energy, and other portions sufiiciently less absorptive of said radiant energy so that, on irradiation of said original as hereinafter provided, only the said highly absorptive portions will attain a temperature sufficient to cause a visible change in an associated heatsensitive copying-surface; said method comprising: (a) placing said graphic original in contact with a separate visibly transparent heat-sensitive copying-paper which exhibits a permanent visible change upon exposure to heat, (5) strongly and briefiy exposing said graphic original to high-intensity light through said visibly transparent copying-paper to provide at said graphic original an elevated-temperature pattern corresponding to the absorptive portions of said graphic original and of an intensity adequate to cause a visible change in said associated copying-paper, and (c) separating said original and said copying-paper.

6. A method for the production of permanent fac- 6 simile copies of a graphic original having a pattern consisting of portions hi hly absorptive of radiant energy, said radiant energy on absorption thereby being converted to heat energy, and other portions sufficiently less absorptive of said radiant energy so that, on irradiation of said original as hereinafter provided, only the said highly absorptive portions will attain a temperature sufficient to cause a visible change in an associated heat-sensitive copying-surface; said method comprising: (a) placing said graphic original in heat-conductive contact with a copying-paper comprising a continuous, firmly bonded, thin, heat-sensitive coating on a support member, said coating being further characterized in that it changes visibly when heated to a temperature within the range of about 50-l50 C., (b) strongly and briefly irradiating said graphic original with said radiant energy to provide an elevated-temperature pattern corresponding with the absorptive portions of said graphic original and of an intensity adequate to cause a visible change in the heatsensitive coating of said copying-paper, and (c) separating the copy from the original.

7. A method for the direct copying of a printed page having infra-red absorptive printed characters, com prising: (a) placing said printed page in close and heatconductive contact with a copying-paper comprising a continuous, firmly bonded and non-otfsetting, thin, heatsensitive particulate waxy coating on a thin cellulosic backing, said coating being further characterized in that it changes visibly when heated to a temperature within the range of about 50-150" (1., (b) briefly exposing the printed surface of said printed page, of which a copy is desired, to intense infra-red radiation, the exposure being suliicient to raise the temperature of the printed characters of said printed page to a point at which the heat-sensitive coating of the contacting copying-paper is visibly changed, and (c) separating the copy from the printed page.

8. The method of preparing copy from an original, comprising the steps of placing the original in surface contact with a copy sheet treated with a material which, responsive to heat, changes color, and directing radiations rich in infra-red on to the original whereby the radiations are absorbed and converted into heat by material in the dark areas of the original, the heat causing a color change to develop in the material with which the copy sheet is treated.

9. The method of preparing copy directly from an original comprising the steps of placing the original in surface contact with the copy sheet coated with a composition which, responsive to heat, causes a color change, directing a sufficient quantity of radiations rich in infrared onto the copy to generate heat for causing the color change to develop in corresponding areas of the copy sheet.

10. A method for the production of permanent facsimile copies of a graphic original having a pattern consisting of portions highly absorptive of radiant energy, said radiant energy on absorption thereby being converted to heat energy, and other portions sufiiciently less absorptive of said radiant energy so that, on irradiation of said original as hereinafter provided, only the said highly absorptive portions will attain a temperature sufficient to cause a visible change in a heat-sensitive copy sheet in surface contact with said original; said method comprising: (a) placing said graphic original in surface contact with a copy sheet treated with a material which, responsive to heat, changes color, and (b) strongly and briefly exposing said graphic original to said radiant energy to provide thereon an elevated-ternperature pattern corresponding to the absorptive portions of said graphic original and of an intensity adequate to develop a permanent visible change in corresponding areas of the copy sheet.

11. A method for the production of permanent facsimile copies of a graphic original having a pattern consisting of portions highly absorptive of radiant energy,

said radiant energy on absorption thereby being converted' to heat energy, and other portions sufficiently less absorptive of said radiant energy so that, on irradiation of said original as hereinafter provided, only the said highly absorptive portions will attain a temperature sufficient to cause a visible change in a heat-sensitive copy sheet in surface contact with said original; said method comprising: (a) smoothly supporting said original and said copy sheet as a two-ply combination in mutually heat-conductive contact, and (I?) strongly and briefly irradiating said original with said radiant energy to pro vide a corresponding localized heat-pattern for producing a visible change in said heat-sensitive copy sheet.

References Cited in the file of this patent UNITED STATES PATENTS 8 1,844,199 Bicknell et a1. Feb. 9, 1932 1,880,449 Hickman et a1. Oct. 4, 1932 1,897,843 Hickman et al. Feb. 14, 1933 2,095,839 Sheppard Oct. 12, 1937 2,103,385 Salfisberg Dec. 28, 1937 2,129,242 Sheppard et a1. Sept. 6, 1938 2,317,789 Marriott Apr. 27, 1943 2,402,631 Hull June 25, 1946 2,519,321 Newman Aug. 15, 1950 2,536,048 Flanagan Jan. 2, 1951 FOREIGN PATENTS 379,333 Great Britain 1932 OTHER REFERENCES Clark: Photography by Infrared, second ed. 1946, John Wiley & Sons, New York, pp. 99 and 112 to 114 cited. 

